I have players try to justify as advantage all the time.Hmm, but what about the other foot? What would you say if a player said “because of XYZ i’m going to have advantage on attacks and skill checks for the duration?”
If it’s OK in the one case is it OK in the other?
“I stand on the table anencephaly leap off. Is that enough to give me advantage?” “I’m filled with fury defending the child from the goblin. Does that give me advantage?” “That orc is standing in slippery mud. Is that enough to give me advantage?”
Sometimes I’ll say “no”. But sometimes I’ll say “yes”.
Which is exactly like what happens when the players asked to give themselves disadvantage and the DM said “yes.”